1. Field of the Invention
The present invention relates to a secondary battery, a battery pack using the secondary battery and a car using the secondary battery or the battery pack.
2. Description of the Related Art
Nonaqueous electrolyte batteries using a lithium metal, lithium alloy, lithium compound or carbonaceous materials as the negative electrode active material are expected as high energy density batteries and earnest studies are being made as to these nonaqueous electrolyte batteries. Lithium ion batteries comprising a positive electrode containing LiCoO2 or LiMn2O4 as an active material and a negative electrode containing a carbonaceous material that absorbs and release lithium ions have been widely put to practical use in portable telephones so far.
In the case of mounting a battery in vehicles or electric trains, on the other hand, materials superior in chemical or electrochemical stability, strength and corrosion resistance are desired as the structural materials of the positive electrode or negative electrode from the viewpoint of storage characteristics at a high temperature (60° C. or more), cycle performance and long term reliability of high output. Moreover, high battery performance is required even in cold districts, which means that it is demanded of the battery to have high-output performance and long-life performance at a temperature as low as about −40° C. In the meantime, a nonvolatile and inflammable electrolytic solution is developed from the viewpoint of improving the safety required for a nonaqueous electrolyte. However, this is accompanied by reductions in output performance, low-temperature performance and long-life performance and therefore, such an electrolytic solution has not been put to practical use.
Therefore, from the foregoing descriptions, lithium ion batteries have large problems concerning high-temperature durability and low-temperature output characteristics to mount them in vehicles and the like. Particularly, it is difficult to mount and use this lithium ion battery in the engine compartment of a car as a substitute for lead-acid batteries.
Various attempts have been made to improve negative electrode characteristics. JP-A 2002-42889 (KOKAI) discloses that a negative electrode having a structure in which a current collector made from aluminum or an aluminum alloy is made to carry a specified metal, alloy or compound is used in a nonaqueous electrolyte secondary battery.
On the other hand, JP-A 2001-143702 (KOKAI) discloses that primary particles of lithium titanate compound represented by the formula LiaTi3-aO4 (0<a<3) and having an average particle diameter less than 1 μm are coagulated into granules having an average particle diameter of 5 to 100 μm to form secondary particles, which are used as a negative electrode active material. Also, in JP-A 2001-143702 (KOKAI), there is the description that the coagulation of secondary particles is suppressed by the use of this negative electrode active material, which increases the production yield of a negative electrode having a large area for a large scale battery.
A remarkable attention is focused on lithium iron phosphate (LixFePO4) which is a lithium phosphorus compound having an olivine crystal structure as a positive electrode active material to improve the performance of the positive electrode and this lithium iron phosphate is expected to improve thermal stability under high-temperature conditions. On the other hand, studies are made to attain low-temperature performance and high-temperature life performance by improving a nonaqueous electrolyte.
However, JP-A 11-329395 (KOKAI) discloses that pores of a macroporous matrix are impregnated with a solution containing a microporous polymer and the resulting macroporous matrix is used as a separator. In the separator disclosed in JP-A 11-329395 (KOKAI), a macroporous of the macroporous matrix is used to support the microporous polymer and has almost no function to support the electrolytic solution. Therefore, the separator described in JP-A 11-329395 (KOKAI) is inferior in the ability of impregnating with the electrolytic solution.
On the other hand, in JP-A 5-74437 (KOKAI), there is the description that a porous film in which a uniform porous coating made of a material having a low hydrogen overvoltage is formed on at least one surface thereof by deposition is used as separater. There is also the description that the surface area of the porous film before the porous coating is formed by deposition is at least 10 m2/g and the average pore diameter of the porous film is about 200 to about 10,000 Å. The separator described in JP-A 5-74437 (KOKAI) uses metal such as nickel as the material having a low hydrogen overvoltage and therefore not only develops short circuits but also is inferior in the ability of impregnating with an electrolytic solution.